Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides - a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fraction as well as naphtha, kerosine, jet fuel, fuel oil, and the like.
A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent-usually air. The prior art shows that catalysts such as metal phthalocyanines and metal porphyrins can be used to oxidize the mercaptans in a liquid/liquid system. See, U.S. Pat. Nos. 2,999,806 and 2,966,453, respectively.
Although the above processes have shown commercial success, there are problems associated with the use of alkaline agents. One problem is that phenols and cresols present in the hydrocarbon stream are extracted into the aqueous alkaline solution. Since phenol is on the EPA list of hazardous compounds, the solution containing the phenols is considered a hazardous waste and must be disposed of according to EPA procedures. Also because of the presence of alkali metals, the aqueous waste stream often cannot be re-used in other parts of the refinery owing to possible contamination of vessels or catalysts with the alkali metals.
Applicants have found that a sour hydrocarbon fraction can be sweetened by using an aqueous solution containing ammonium hydroxide, an onium compound and a metal chelate. Onium compounds are quaternary ammonium compounds, phosphonium compounds, arsonium compounds, etc. A preferred onium compound is a quaternary ammonium compound with a preferred quaternary ammonium compound being a quaternary ammonium chloride. Quaternary ammonium compounds, and onium compounds in general, have been used in sweetening sour hydrocarbon fractions in conjunction with fixed bed catalysts, e.g., a metal phthalocyanine deposited on an activated charcoal. See, e.g., U.S. Pat. Nos. 4,156,641, 4,124,494, 4,260,479 and 4,203,827. Further, U.S. Pat. No. 4,923,596 discloses the use of quaternary ammonium compounds in a liquid/liquid system using a caustic solution.
Finally, the use of ammonium hydroxide has been disclosed in the prior art, but only as it relates to a fixed bed system. For example, U.S. Pat. No. 4,913,802 discloses the use of ammonium hydroxide and a quaternary ammonium hydroxide in conjunction with a metal chelate dispersed on a support. Similarly, U.S. Pat. No. 4,908,122 discloses the use of ammonium hydroxide and a quaternary ammonium salt other than hydroxide in conjunction with a metal chelate dispersed on a support.
There is no hint nor suggestion in any of the prior art that an aqueous solution containing a metal chelate, ammonium hydroxide and an onium compound can be used to sweeten a sour hydrocarbon fraction. Although U.S. Pat. No. 2,966,453 mentions that ammonium hydroxide can be used to prepare a solution of a metal porphyrin (column 4, lines 69-71), there is no hint or suggestion that such a solution could be used to treat a sour hydrocarbon fraction to give a doctor sweet product. It is applicants alone who have found that ammonium hydroxide can be substituted for an alkaline solution and still yield a doctor sweet product. In particular, applicants have found that a quaternary ammonium chloride salt has much better activity than a quaternary ammonium hydroxide salt, an unexpected result based on the teachings of the prior art.